Non-probability sampling is a sampling technique in which the researcher selects samples based on the subjective judgment of the researcher rather than random selection.
In non-probability sampling, not all members of the population have a chance of participating in the study unlike probability sampling, where each member of the population has a known chance of being selected.
Non-probability sampling is most useful for exploratory studies like pilot survey (a survey that is deployed to a smaller sample compared to pre-determined sample size). Non-probability sampling is used in studies where it is not possible to draw random probability sampling due to time or cost considerations.
Non-probability sampling is a less stringent method, this sampling method depends heavily on the expertise of the researchers. Non-probability sampling is carried out by methods of observation and is widely used in qualitative research.
Types of non-probability sampling
Convenience sampling is probably the most common of all sampling techniques. With convenience sampling, the samples are selected because they are accessible to the researcher. Subjects are chosen simply because they are easy to recruit. This technique is considered easiest, cheapest and least time consuming.
Consecutive sampling is very similar to convenience sampling except that it seeks to include ALL accessible subjects as part of the sample. This non-probability sampling technique can be considered as the best of all non-probability samples because it includes all subjects that are available that makes the sample a better representation of the entire population.
Quota sampling is a non-probability sampling technique wherein the researcher ensures equal or proportionate representation of subjects depending on which trait is considered as basis of the quota.
Judgmental sampling is more commonly known as purposive sampling. In this type of sampling, subjects are chosen to be part of the sample with a specific purpose in mind. With judgmental sampling, the researcher believes that some subjects are more fit for the research compared to other individuals. This is the reason why they are purposively chosen as subjects.
Snowball sampling is usually done when there is a very small population size. In this type of sampling, the researcher asks the initial subject to identify another potential subject who also meets the criteria of the research. The downside of using a snowball sample is that it is hardly representative of the population.
When to use non-probability sampling?
- This type of sampling is used to indicate if a particular trait or characteristic exists in a population.
- This sampling technique is widely used when researchers aim at conducting qualitative research, pilot studies or exploratory research.
- Non-probability sampling is used when researchers have limited time to conduct researcher or have budget constraints.
- Non-probability sampling is conducted to observe if a particular issue needs in-depth analysis.
Advantages of Non-Probability Sampling
- Possibility to reflect the descriptive comments about the sample
- Cost-effectiveness and time-effectiveness compared to probability sampling
- Effective when it is unfeasible or impractical to conduct probability sampling
Disadvantages of Non-Probability Sampling
- Unknown proportion of the entire population is not included in the sample group i.e. lack of representation of the entire population.
- Lower level of generalization of research findings compared to probability sampling.
- Difficulties in estimating sampling variability and identifying possible bias.